Method of preparing magnetic powder of cobalt-substituted gamma-ferric oxide

ABSTRACT

The present invention relates to improved cobalt-substituted gamma -ferric oxide magnetic powder suitable for application in magnetic recording. Said powder comprises a batch composition which is characterized by addition of controlled amounts of at least one compound selected from the group of NH4, Cu, Li, Pb, Sn, Ba, Cd, Sr and In halide compounds to the mixture of acicular powder of goethite with a controlled amount of cobalt compound, said batch composition being mixed, heated at a temperature from 300* to 600*C in a reducing gas atmosphere, and finally oxidized at a temperature from 150*C to 350*C in air or oxygen gas atmosphere.

United States Patent [191 Matsumoto et al.

[ 1 Sept. 2, 1975 [75] Inventors: Keiji Matsumoto, Osaka; Yoshihiro Matsuo, Neyagawa, both of Japan [73] Assignee: Matsushita Electric Industrial Co.,

Ltd., Japan [22] Filed: Sept. 7, 1973 [21] Appl. No.: 394,998

[52] US. Cl. 252/6258; 252/6256; 252/6259; Y

[51 Int. Cl C04b 35/00 [58] Field of Search 252/6251, 62.56, 67.58, 252/6259, 62.6, 62.61, 62.62, 62.63

[56] References Cited UNITED STATES PATENTS 3,075,919 l/l963 Gruber et al. 252/6262 3,399,142 8/1968 Conley 252/6256 Primary Exuminer.lack Cooper Attorney, Agent, or Firm-Wenderoth, Lind & Ponack 57 ABSTRACT The present invention relates to improved cobaltsubstituted y-ferric oxide magnetic powder suitable for application in magnetic recording. Said powder comprises a'batch composition which is characterized by addition of controlled amounts of at least one compound selected from the group of NH Cu, Li, Pb, Sn, Ba, Cd, Sr and In halide compounds to the mixture of acicular powder of goethite with a controlled amount of cobalt compound, said batch composition being mixed, heated at a temperature from 300 to 600C in a reducing gas atmosphere, and finally oxidized at a temperature from 150C to 350C in air or oxygen gas atmosphere.

14 Claims, 3 Drawing Figures PATENTED SEP 2 I 75 Coercive Force Ho (e) 3,903,004 SHEET 1 OF 3 30o No. 4

No. l

Temperature (C) Fig. 1

PATENTEUSEP 21975 S'nLET 2 [IF 3 A Q 0 v o m 400 No. 14

8 No.16 H No. 66 o No.92 300 No.74 No. 60 g No. 323 1) No.46 o O 200 No. l

O I l I Temperature (C) Fig. 2

PATENTEU sum 3 mg 3 $8 om munc $53,300

Temperature (C) Fig. 3

METHOD OF PREPARING MAGNETIC POWDER OF COBALT-SUBSTITUTED GAMMA-FERRIC OXIDE BACKGROUND OF THE INVENTION This invention relates to magnetic powder, and more particularly to y-ferric oxide magnetic powder suitable for application in magnetic recording tapes with the provision of improved storage temperature characteristics and to a method for manufacturing the same.

It has heretofore been suggested that cobalt-substituted y-ferric oxide could be used in the manufacture of magnetic recording tapes, particularly those intended to record video signals. Cobalt-substituted 'yferric oxide has a high coercive force and satisfactory electromagnetic conversion characteristics such as frequency response, signal to noise ratio, and the like. However, magnetic tapes employing cobalt-substituted y-ferric oxide magnetic powder exhibit poor storage temperature characteristics which arise from a large temperature dependency of the coercive force of cobalt-substituted -y-ferric oxide magnetic powder.

This disadvantage is not shared by some of the magnetic powders such as chromium dioxide, CrO Magnetic tapes made with such magnetic powders have the advantage over cobalt-substituted y-ferric oxide magnetic powders in that the former is substantially independent of storage temperature characteristics. Consequently, if the cobalt-substituted yferric oxide system Could be modified so that its coercive force is no longer dependent upon temperature changes, this material would be effectively competitive with. the chromium dioxide.

SUMMARY OF THE INVENTION An object of this invention, therefore, is to provide cobalt-substituted 'y-ferric oxide magnetic powder having a high coercive force and small temperature dependency of coercive force.

Another object of this invention is to provide cobaltsubstituted 'y-ferric oxide magnetic powder having a high coercive force, a high rcmanent induction, and small temperature dependency of coercive force.

Another object of this invention is to provide the method of manufacturing cobalt-substituted 'y-ferric oxide magnetic powder having a high coercive force, and small temperature dependency of coercive force.

These objects are achieved by providing an improved cobalt-substituted y-ferric oxide magnetic powder according to the present invention. which comprises a batch composition which consists essentially of 0.425 to 9.95 mol. 7: ofcobalt compound, 0.5 to mo].% of at least one compound selected from the group consisting of NH Cl, CuCl, LiCl, PbCl SnCl- BaCl CdCl SrCl and Incl and 75.05 to 99.075 mol.% of acicular powder of goethite. Said cobalt compound is at least one member selected from the group consisting of C050,, CoCl Co(NO;,) and Co(CH;,COO) which is soluble in water. Said compound selected from said group of chloride can have Br. I or F substituted for CI.

A method of preparing the improved cobalt-substituted -y-ferric magnetic powders is as follows. Acicular powder of goethite is dispersed into an aqueous solution of cobalt compound such as CoSo,,. A precipitating reagent such as NaOH is added to the aqueous solution being stirred in order to precipitate all of the cobalt ions in the form of cobalthydroxide. Then, the

mixture of goethite and cobalt hydroxide is filtered, washed, dried, further mixed with at least one compound selected from the group of haloid compounds. The final mixture of goethite, cobalt hydroxide, and haloid is heated at a temperature from 300C to 600C in a reducing gas atmosphere, and finally oxidized at a temperature from C to 350C in air or oxygen gas atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the invention will be apparent upon consideration of following detailed description taken together with accompanying drawing wherein:

FIG. I is a graph showing temperature dependency of coercive force with a parameter of additive amounts of CuCl for cobalt substituted 'y-ferric oxide.

FIG. 2 is a graph showing a temperature dependency of coercive force for eoventional cobalt-substituted 'yferric oxide and improved cobalt-substituted 'y-ferric oxide with some additives.

. FIG. 3 is a graph showing a temperature dependency of rcmanent induction for cobalt-substituted 'y-ferric oxide added with some haloid compounds.

DETAILED DESCRIPTION OF THE INVENTION Improved cobalt-substituted 'y-ferric oxide magnetic powder according to the present invention comprises a batch composition which consists essentially of 0.425 to 9.95 mol.% of cobalt compound, 0.5 to 15 mol.% of at least one compound selected from the group consisting of NH ,Cl, CuCl, LiCl, PbCl SnCl BaCl CdCl SrCl and InCl and 75.05 to 99.05 mol.% of acireular powder of goethite.

The compositions claimed can not be defined by the composition finally obtained after heat-treatment, but by the batch composition as a starting mixture before heat-treatment.

The reason is due to the difficulty of determining exactly the final composition, because crystallographic sites which added metal ions and added chlorine ions occupy in crystal lattice of 'y-Fe o is unknown and because vaporization of some of added chlorine during heat treatment occurs thereby changing the composition.

Said cobalt compound is desirably water-soluble such as C050,, CoCl- Co(NO;,)- and Co(CH,,COO)

Said chlorine ions in said compound selected from the said group can be substituted by bromine, iodine, or fluorine. That is, the compounds are NH ,Br, CuBr, LiBr, PbBr SnBr BaBr CdBr SrBr InBr NH I, Cul, LII, Pbl Snl Cdl Srl- Inl NH ,F, CuF, LiF, PbF SnFZ, BaF CdF SrF or MB.

A method of manufacturing improved magnetic powder according to the present invention comprises essentially of dispersing acicular powder of goethite into an aqueous solution of said cobalt compound, precipitating all of the cobalt ions in a form of cobalt hydroxide by addition of a percipitating reagent such as NaOH aqueous solution into the aqueous solution of said cobalt compound, filtering the mixture of goethite and cobalt hydroxide, drying the mixture, mixing at least one compound selected from the said group of haloid compounds with the mixture of goethite and cobalt hydroxide, heating the final mixture at a temperature from 300C to 600C in a reducing gas atmosphere such as H N mixed gases, and finally oxidizing the final mixture at a temperature from 150C to 350C in 3 air or oxygen gas atmosphere.

Said precipitated cobalt hydroxide is adjusted to an atomic percent of 0.5 to 10.0% of Co and 90.0 to 99.5% of Fe.

The total amount of said one compound selected from the said group of haloid is 0.5 to molF/r in respect to 85 to 99.5 mol.% of mixture of goethite and cobalt hydroxide.

This corresponds to a batch composition consisting of 0.425 to 9.95 mol.% of cobalt compound, 0.5 to 15 mol.% of haloid compound, and 75.05 to 99.075 molT/r of goethitc EXAMPLES Additions of many chloride compounds were attempted in preparation of cobalt-substituted y-ferric oxide. 500 gram of acicular powders of goethite with the size of 0.4 to 0.6 am in length and 0.06 to 0.08 ,u.rn in width were dispersed into 10 liters of aqueous solution containing 26.9 gram of CoSO Their amounts are in the atomic ratio of 97 atomic Fe and 3 atomic Co. Ammonia water was added to the goethite-dispersed aqueous solution of CoSO until the pH of the aqueous solution became more than 10, and then stirring of the solution was maintained for ten minutes. It resulted that all of the cobalt ions in the solution were precipitated in the form of cobalt hydroxide, The mixtures of 97 mol.% of goethite and 3 mol.% of cobalt hydroxide were filtered, washed with pure water, and dryed at a temperature of 90C for 6 hours.

Then, one compound selected from the chloride group was added to a 10g batch of the mixture of goethite and cobalt hydroxide. The added amounts of said one compound are shown in the following parentheses; the former is in the molar ratio of 3 mo1.7c of chloride and 97 mol.% of the mixture of goethite and cobalt hydroxide, and the later is in the molar ratio of 10 mol.% of chloride and 90 mol.% of the mixture of goethite and cobalt hydroxide. The said chloride group consists of NH Cl (0.186g and 0.668g), CuCl (0.344g and 1.236), LiCl (0.147g and 0,529g), PbC1 (0.967g and 3.473g), SnC1- (0.659g and 2.368g), BaCl (0.724g and 2.600g), CdCl (0.637g and 2.889g), SrCl (0.510g and 1.980), lnCL, (0.769g and 2.762g), AgCl (0.498g and 1.790g), KCl 0.259g and 0.931g), NaCl (0.203g and 0.730g), NiC1 (0.451g and 1.6l9g), PdC1 (0.616g and 2.214g), ZnCl (0.474g and 1.702g), CaC1 (0.386g and 1.386g), MgCl (0.33 lg and 1.189g), MnCl (0.437g and 1.57 1 g), YCL, (0.679g and 2.438g), GdCL, (0.916g and 3.292g), and LaCL (0.853g and 3.063g). The batch compositions consisting of goethite, cobalt hydroxide, and chloride compound were intimately mixed with a small amount of pure water using agate mortar, and then dryed at a-tempcrature of 90C for 2 hours to be free from water.

The final mixtures of the batch composition were heated at a temperature of 450C for 1 hour in the atomosphere of 1071 H. ,-907(N "as flow. During the heat treatment, goethite a-FcOOl-l) was converted to magnetite (Fe O containing C0, C1, and the other added ions.

Then, the magnetite powders were heated up to 250C at the rate of 100C/H in air and kept at 250C for 5 hours in air to be converted to maghemetite (y- Fe- O containing C0, C1, and the other added ions.

For each sample, the coercive force He and remanent induction Br were determined from measurements 4 of 8-H curve at different temperatures, and shown in Table 1 and Table 2.

A group of additives in Table 1 have effects to increase coercive force and decrease a temperature dependency of coercive force. All of the values of the coercive force at 25C are larger than 380 Oe, which is the value of coercive force at 25C for -y-(Fe,, Co,, O; powders with no additive. On the other hand, a group of additives in Table 2 have no effect to increase coercive force.

Table 3 shows experimental results with variation of added amounts of CuCl from 0.2 mol.% to 20 mol.%, fixing doped amounts of Co to be 3 atomic 7c. Table 4 and Table 5 are in cases of variation of added amounts of PbCl and lnC1 respectively, with a fixed 3 atomic of Co. All of the samples were prepared in the same process as mentioned above. As seen in these tables, additions of CuCl, PbC1 and lnCl to be from 0.5 mol.% to 15 mol.% have the effect of increasing coercive force and decreasing temperature dependency of the coercive force. Also, in cases of the other additives shown in Table 1 the same effects were observed in the added amounts from 0.5 mol.% to 15 mo1./ The beneficial effects of CuCl additions are shown in FIG. 1.

Next, examples were run with variation of doped amounts of Co from 0.2 atomic to 15 atomic "/0, fixing added amounts of CuCl to be 3 mol.% and 10 mol.%, shown in Table 6 and Table 7, respectively. The C0 amount more than 0.5 atomic is favorable to obtain the coercive force larger than 380 Oe. The C0 amount smaller than 10 atomic is favorable to obtain a small temperature dependency of coercive force, that is more than about of the ratio coercive force at C to that at 25C. This limitation to Co amounts was applied in cases of the other additions in Table 1.

Next, examples were run with substitution of F, Br, and 1 ions for C1 ion of eight kinds of additives in Table 1, fixing doped amounts of Co to be 3 atomic /6. All of the halogen additives, shown in Table 8, have the effect of increasing the coercive force to more than 380 Oe and to decreasing temperature dependency of the coercive force. The beneficial effects of some additives are shown in FIG. 2.

Table 9 shows experimental results with variation of added amounts of copper bromide, copper iodide, and copper flouride from 0.2 moL /L to 20 mol /1., fixing doped amounts of cobalt to be 3 atomic 7r. All of the sample were prepared in the same process as mentioned above. As seen in this table, additions of CuBr, Cul, and CuF from 0.5 mol /r to 15 mol.% have the ef' fect of increasing coercive force and decreasing temperature dependency of the coercive force. Also, in case of the other additives shown in Table 8, the same beneficial effects were observed in the added amounts from 0.5 mo1./( to 15 11101.7(. Furthermore, the beneficial effects were observed in the limited amount of doped Cobalt ion to be from 0.5 atomic to 10 atomic All of the claimed additives showed a higher remanent induction (Br) than 1850 gauss. Some curves showing a temperature dependency of the rcmanent induction are given in FIG. 3.

Outstanding examples of the present invention are the following; No. 1, No. 20No. 43, No. 121, N0. 129 No. 141, No. 149, No. 191, No. 199, No. 221, No. 228, No. 321, No. 328, No. 421, No. 429, No. 721 No. 729, No. 1021, and No. 1029.

Tah1c'8-continucd Sample Additives m 'y- Added (ocrcivc force Hc(Oc Rcr'nuncnt induction Br(guuss) Nu. (Fc.. ,,;(.u amount I (muW' C 80C 1 C HC( 80C) 25C 80C 150C Br( 80C) Hc( 25C) Br( 25C) 51 I0 400 330 240 0.83 2030 1950 1720 0.96 52 CuI 3 460 370 270 0.80 2050 1920 1730 0.94 53 10 450 360 250 0.80 2090 I940 1850 0.93 54 CuF 3 480 360 270 0.75 2050 1930 1820 0.94 55 10 430 340 250 0.79 2030 1900 1790 0.94 56 LiBr 3 480 410 270 0.85 2080 1970 1890 0.95 57 10 400 350 240 0.88 2050 I970 1870 0.96 58 1.11 3 390 320 270 0.82 2070 1960 1830 0.95 59 I0 390 310 260 0.79 2030 1930 1800 0.95 60 [.IF 3 480 400 270 0.83 2090 I970 1860 0.94 61 10 4 I 0 340 240 0.83 2050 1950 1850 0.95 62 PbBr 3 530 430 320 0.81 2090 1980 18 30 0.95 63 I0 390 330 280 0.85 2090 1970 1820 0.94 64 Ph1. 3 570 450 340 0.79 2 I 30 2000 1860 0.94 65 10 400 330 270 0.83 2090 1960 1830 0.94 66 PhF- 3 620 470 350 0.76 2090 2000 1940 0.96 67 10 450 390 310 0.87 2150 2030 1900 0.94 68 SnBr: 3 430 340 230 0.79 2090 1990 1870 0.95 69 10 380 300 200 0.79 2200 2080 1960 0.95 70 SnI- 3 390 310 210 0.79 2200 2050 1940 0.93 71 10 430 330 220 0.77 1970 1860 1770 0.94 72 SnF 3 420 320 220 0.76 2400 2160 2030 0.90 73 10 400 310 210 0.78 2200 2020 1930 0.92 74 BzlBr 3 500 400 300 0.80 2200 2040 1930 0.93 75 10 390 310 220 0.79 2 I 2020 1900 0.93 76 13:11;- 3 390 320 2 I 0 0.82 2 I 40 2010 1910 0.94 77 10 370 300 200 0.79 2100 1990 1910 0.95 78 BuF 3 400 320 230 0.80 2340 2 I 80 2060 0.93 79 I0 380 310 200 0.82 2180 2000 1890 0.92 80 C0Br- 3 520 420 310 0.81 2130 2010 1940 0.94 81 I0 410 310 220 0.76 1980 1870 1790 0.94 82 Cdl: 3 470 360 250 0.77 2160 2070 1990 0.96 83 10 420 320 210 0.76 2050 1940 1850 0.95 84 CLIP-1 3 600 490 350 0.82 2160 2000 1920 0.93 85 I0 400 310 220 0.78 2040 1930 1820 0.95 86 SrBr- 3 450 340 240 0.76 2100 1980 1870 0.94 87 10 390 300 210 0.77 2020 1950 1820 0.97 88 SH: 3 500 410 280 0.82 2200 1990 1800 0.90 89 10 420 320 200 0.76 1980 1880 1760 0.95 90 SrF 3 540 430 300 0.80 2100 1980 1900 0.94 91 10 410 330 220 0.80 2100 1990 1850 0.95 92 InBr 3 500 420 310 0.84 2190 2020 1920 0.92 93 10 420 330 280 0.79 2150 2000 1890 0.93 94 1111:, 3 400 320 200 0.80 2190 2010 1940 0.92 95 10 390 300 200 0.78 2040 1920 1800 0.94 96 InF 3 430 330 290 0.77 2250 2100 1980 0.93 97 10 390 310 200 0.79 1900 1860 1740 v 0.98

Table 9 SampIc Additives to 'y- Added Coercive form: He (00) Rcmancnt induction Br(gauss) Nu. Fc Cn O amount (muV/z 25C 80C 1 50C I"1C( 80C) 25C 80C 1 50C Br( 80C) Hc( 25C) Br 25C) 1 ('uBr 0 380 290 190 0.76 1930 1830 1590 0.95 -12 1 0.2 310 250 190 0.81 2010 1870 1660 0.93 -122 0.5 350 300 250 0.86 2040 1950 1730 0.96 423 1.0 370 310 260 0.84 2070 1970 1750 0.95 80 3.0 510 420 300 0.82 2050 1960 1740 0.96 81 10.0 400 330 240 0.83 2030 1950 1720 0.96 428 15.0 420 340 250 0.81 2060 1920 1700 0.93 429 20.0 290 240 190 0.83 2000 1820 1640 0.91 721 ('ul 0.2 320 250 180 0.78 2010 1860 1660 0.93 722 0.5 370 320 260 0.86 2030 1950 1730 0.96 723 1.0 400 340 250 0.85 2030 1890 1700 0.93 82 3.0 460 370 270 0.80 2050 1920 1730- 0.94 83 10.0 450 360 250 0.80 2090 1940 1850 0.93 728 15.0 480 380 270 0.79 2000 1880 1680 0.94 729 20.0 360 270 220 0.75 1920 1720 1580 0.90 1021 ('uF 0.2 290 210 170 0.72 2020 1850 1570 0.92 1022 0.5 360 290 200 0.81 2100 1950 1840 0.93 1023 1.0 420 320 230 0.76 2080 1960 1860 0.94 84 3.0 480 360 270 0.75 2050 1930 1820 0.94 85 10.0 430 3-10 250 0.79 2030 1900 1790 0.94 1028 15.0 4110 300 200 (1.75 2030 1900 1800 (1.94 102) 211.0 29(1 21(1 (1.72 1970 1850 1020 0.94

I. A method of preparing a magnetic powder consist- Whm. i claimed i ing mainly of cobalt-substituted 'y-fcrric oxide. which essentially comprises dispersing aeicular powder of goethite into an aqueous solution of a cobalt compound. precipitating all of the cobalt ions in the form of cobalt hydroxide by addition of a precipitating reagent into the aqueous solution of cobalt compound, filtering the resultant mixture of goethite and cobalt hydroxide, the atomic percent of Fe in said goethite-cobalt hydroxide mixture being 90.0 to 99.5 and the atomic "/1 of Co being 0.5 to 10.0, mixing 0.5 to mol. 7! in total, of at least one halide selected from the group consisting of NH X, CuX, LiX, PbX- SnX. BaX CdX. SrX and lnX wherein X is F, Cl, Br or I, with 85 to 99.5 mol. 7(- of the mixture of gocthite and cobalt hydroxide. heating the final mixture at a temperature from 300C to 600C in a reducing gas atmosphere, and finally oxidizing the final mixture at a temperature from 150C to 350C in air or oxygen gas atmosphere.

2. A method as claimed in claim 1, wherein X is Br.

3. A method as claimed in claim 1, wherein X is l.

4. A method as claimed in claim 1, wherein X is F.

5. A method as claimed in claim 1, wherein said cobalt compound in an aqueous solution is at least one member selected from the group consisting of C050 CoCl Co(NO;,) and Co(CH;,COO).

6. A method as claimed in claim 1, wherein said halide is NH CI.

7. A method as claimed in claim 1, wherein said halide is CuCL 8. A method as claimed in claim I, wherein said halide is LiCl.

9. A method as claimed in claim I, wherein said halide is PbCl- 10. A method as claimed in claim 1, wherein said halide is SnCL.

11. A method claimed in claim 1, wherein said halide is BaCl 12. A method as claimed in claim 1, wherein said halide is SrCl 13. A method as claimed in claim 1, wherein said halide is lnCl;,.

14. A method as claimed in claim 1, wherein X is Cl. 

1. A METHOD OF PREPARING A MAGNETIC POWDER CONSISTING MAINLY OF COBALT-SUBSTITUTED Y-FERRIC OXIDE, WHICH ESSENTIALLY COMPRISES DISPERSING ACICULAR POWDER OF GOETHITE INTO AN AQUEOUS SOLUTION OF A COBALT COMPOUND,PRECIPITATING ALL OF THE COBALT IONS IN THE FORM OF COBALT HYDROXIDE BY ADDITION OF A PRECIPITATING REAGENT INTO THE AQUEOUS SOLUTION OF COBALT COMPOUND, FILTERING THE RESULTANT MIXTURE OF GOETHITE AND COBALT HYDROXIDE, THE ATOMIC PERCENT OF FE IN SAID GOETHITE-COBALT HYDROXIDE MIXTURE BEING 90.0 TO 99.5 AND THE ATOMIC % OF CO BEING 0.5 TO 10.0, MIXING 0.5 TO 15 MOL. % IN TOTAL, OF AT LEAST ONE HALIDE SELECTED FROM THE GROUP CONSISTING OF NH4X, CUX, LIX, PBX2, SNX2, BAX2, SRX2 AND INX3, WHEREIN X IS F, C1, BR OR 1, WITH 85 TO 99.5 MOL. % OF THE MIXTURE OF GOETHITE AND COBALT HYDROXIDE, HEATING THE FINAL MIXTURE AS A TEMPERATURE FROM 300*CTO 6000*C IN A REDUCING GAS ATMOSPHERE, AND FINALLY OXIDIZING THE FINAL MIXTURE AT A TEMPERATURE FROM 150*C TO 350*C IN AIR OR OXYGEN GAS ATMOSPHERE.
 2. A method as claimed in claim 1, wherein X is Br.
 3. A method as claimed in claim 1, wherein X is I.
 4. A method as claimed in claim 1, wherein X is F.
 5. A method as claimed in claim 1, wherein said cobalt compound in an aqueous solution is at least one member selected from the group consisting of CoSO4, CoCl2, Co(NO3)2, and Co(CH3COO)2.
 6. A method as claimed in claim 1, wherein said halide is NH4Cl.
 7. A method as claimed in claim 1, wherein said halide is CuCl.
 8. A method as claimed in claim 1, wherein said halide is LiCl.
 9. A method as claimed in claim 1, wherein said halide is PbCl2.
 10. A method as claimed in claim 1, wherein said halide is SnCl2.
 11. A method as claimed in claim 1, wherein said halide is BaCl2.
 12. A method as claimed in claim 1, wherein said halide is SrCl2.
 13. A method as claimed in claim 1, wherein said halide is InCl3.
 14. A method as claimed in claim 1, wherein X is Cl. 